In the large scale production of alkanesulfonic acids by oxidation of mercaptans or dialkyl disulfides, it is desirable but extremely difficult to achieve complete oxidation of the odorous organosulfur impurities without oxidation of the product acid. Such overoxidation can result in the formation of undesired sulfate, whereas underoxidation allows small amounts of the undesirable odorous oxidizable sulfur compounds to remain. Such compounds include the starting mercaptans (RSH) and dialkyl disulfides (RSSR') as well as alkyl alkanethiolsulfonates (RSO.sub.2 SR'), alkyl alkanethiolsulfinates (RS(O)SR') and dialkyl sulfoxides (RSOR'), where R and R' are alkyl groups and R and R' can be the same or different. These and other impurities may cause significant unpleasant odor and lead to the formation of color. Further odor or color may develop when the alkanesulfonic acid or its aqueous solution is subsequently used as a reactant or solvent.
A number of methods of purifying sulfonic acids and their salts are known which involve treatment with various chemicals including nitric acid, cyanuric acid, sodium hypochlorite, hypobromate and hydrogen peroxide. These methods have the disadvantage of introducing new impurities into the sulfonic acid. These impurities must be removed, in some cases, such as when nitric acid is used, with great difficulty.
Honeycutt discloses the improvement of the color and wetting properties of sulfonic acid salts through contact with ozone, which has the advantage of not introducing a new impurity to the mixture being treated. The salts treated by Honeycutt are contained in petroleum sulfonate stocks derived from the sulfonation of petroleum. The color causing materials in such stocks would be expected to differ from the organosulfur impurities resuling from the formation of alkane sulfonic acids by the oxidation of mercaptans and disulfides.
Ozone has been disclosed as an initiator for the sulfoxidation of saturated hydrocarbons by Bost et al in U.S. Pat. Nos. 3,413,337, 3,479,398 and 3,485,870 to permit the use of lower reaction temperatures (below 50.degree. C.) during the sulfoxidation process using a metal, halogen oxide, halogen oxy acid or nitrogen oxide catalyst. Thie process produces sulfuric acid as a side reaction. Ozone is reported to effect some decolorizing of the product sulfonic acids. The nature of the color causing materials is not disclosed but the byproduct sulfuric acid itself is known to cause color by the oxidation or charring of other impurities. As in the case of the Honeycutt process, such color producing compounds would be expected to differ from the organosulfur impurities derived from the oxidation of mercaptans and disulfides.
Two research papers discuss the ozonation or organic sulfides. Barnard [J. Chem. Soc., 4547-4555 (1957)] discloses the oxidation of organic mono-, di- and tetrasulfides with ozone. Barnard reported that dimethyl disulfide was converted to methyl methanethiolsulphonate under his reaction conditions (-25.degree. C.), rather than to the methanesulphonic acid anhydride, because the methanethiol-sulfonate did not further react with ozone. Ericson and Yates (PB-257891 August 1976) in their report of studies on the reaction kinetics of ozone with sulfur compounds disclose that ozone treatment of methanethiol at low concentrations at 0.degree. C. produced methanesulfonic acid along with dimethyldisulfide, methyl methanethiolsulfonate and methyl methanethiolsulphinate as minor constituents and that continued ozonation resulted in very slow formation of sulfuric acid by oxidation of the acid. The above organic side products were not detected when excess ozone was present but any minor products may escape detection due to the low concentrations. Ericson and Yates indicate that with excess ozone the disulfide and thiolsulphinate would have been oxidized but they refer to Barnard's report that thiolsulfonates are resistant to ozonation.
In the large scale production of methanesulfonic acid by catalytic oxidation of methylmercaptan (methanethiol) or dimethyl disulfide, methyl methanethiosulfonate is a significant impurity which must be reduced in concentration or completely removed. Based on the above reports one would expect that ozonation would either fail to oxidize this impurity or, if more rigorous conditions were used in order to attempt to oxidize the thiolsulfonate, at the sulfonic acid product concentrations needed for a practical purification process oxidation of some of the product acid to sulfuric acid would be expected to occur. Surprisingly, I have found that the thiolsulfonate content of alkanesulfonic acids can be significantly reduced (along with other organosulfur impurities) by treatment with ozone, at temperatures of 20.degree. C. and above and sulfonic acid concentrations of 10% or more without significant conversion of sulfonic acid to sulfuric acid.